This application incorporates by reference the subject matter of Application No. 2002-329075, filed in Japan on Nov. 13, 2002, on which a priority claim is based under 35 U.S.C. xc2xa7119(a).
(1) Field of the Invention
This invention relates to an air intake structure of an engine.
(2) Description of the Related Art
As a technique for increasing volumetric efficiency in an air intake structure of an engine, a variable intake structure is known which uses a resonator effect in the low revolution speed region of the engine and uses an inertia effect in the high revolution speed region of the engine.
In the case of a variable intake structure of a V type 6-cylindered engine, for example, 6 intake pipes corresponding to the respective cylinders are gathered, in groups of 3 intake pipes, on an upstream side to form 2 ports, and these two ports are constituted independently as far as the neighborhood of a throttle body with the use of a partition structure. By appropriately setting the length and thickness of each port as an intake air passage, volumetric efficiency is increased with the aid of a resonator effect. A selector valve capable of establishing communication between these two ports is provided at a site of the independently constituted two ports which is immediately before their branching into a plurality of intake pipes in correspondence with the respective cylinders. The two ports, rendered independent, are brought into communication at this site, whereby intake air pulsation is counteracted to obtain an inertia effect. The selector valve is opened or closed according to the engine revolution speed. That is, the two ports are brought into a communication state or a non-communication state, whereby better volumetric efficiency can be obtained when the engine revolution speed is in a low revolution speed region and a high revolution speed region. This can be seen in detail from FIG. 5. Assume, here, that the engine revolution speed is close to the point of contact between a graph, which shows the relationship between the engine revolution speed and the volumetric efficiency when the selector valve is closed, and a graph, which shows the relationship between the engine revolution speed and the volumetric efficiency when the selector valve is open. If, at this time, the selector valve is controlled so as to become open from the closed state, a better volumetric efficiency can be obtained because of the resonator effect of intake air in the low engine revolution speed region and the inertia effect of intake air in the high engine revolution speed region.
In the foregoing variable intake structure, however, when the engine revolution speed is in a revolution speed region where the selector valve is switched from the closed state to the open state, the volumetric efficiency markedly drops (see FIG. 5). That is, the volumetric efficiency due to the resonator effect in the closed state of the selector valve, and the volumetric efficiency due to the inertia effect in the open state of the selector valve both decrease in the engine revolution speed region where the selector valve is switched (i.e. intermediate revolution speed region). Thus, a drop portion occurs between the peaks of the volumetric efficiencies ascribed to the respective effects. This poses the problem of difficulty in increasing volumetric efficiency in this intermediate revolution speed region.
Among techniques for preventing such a drop in volumetric efficiency is the technique disclosed in Japanese Patent Publication No. 1995-39812. According to this technique, in a V type 6-cylindered internal combustion engine, an opening/closing valve 23 is controlled so as to be closed in the low speed operation region of the engine to constitute two resonate charging systems; the opening/closing valve 23 is controlled so as to open, and simultaneously pipe length selector valves 38l, 38r are controlled so as to be closed, in the intermediate speed operation region of the engine to constitute an inertia charging system of a large pipe length where resonance chambers Cr-l, Cr-r form ends open to the air; and the opening/closing valve 23 and the pipe length selector valves 38l, 38r are both controlled so as to open in the high speed operation region of the engine to bring intermediate portions of distribution pipes 351 to 356 into communication with a pipe length changeover chamber Cc, which is a substantial end open to the air, thereby constituting an inertia charging system of a small pipe length. In this manner, the resonate charging system is constituted in the low speed operation region, and the inertia charging systems of large and small pipe lengths are constituted in the intermediate and high speed operation regions, respectively. By so doing, the volumetric efficiency is increased over the broad operating region of the engine (see patent document 1, FIG. 1).
With the technology described in the above publication, however, the inertia charging system is constituted in the intermediate and high speed operation regions of the engine. This has presented the problem that the structure of the entire air intake system is very complicated.
The present invention has been accomplished in the light of the above-mentioned problems. It is the object of the invention to provide an air intake structure of an internal combustion engine, which has a relatively simple configuration and has a satisfactory volumetric efficiency in the broad operation region of the engine.
The air intake structure of an engine according to the present invention comprises a first intake air passage and a second intake air passage formed by branching of an intake air passage disposed downstream from a throttle valve; a plurality of intake ducts branched from a downstream end portion of the first intake air passage and a downstream end portion of the second intake air passage and having a downstream side connected to a plurality of cylinders provided in the engine; a first communication element capable of establishing communication between the neighborhood of the downstream end portion of the first intake air passage and the neighborhood of the downstream end portion of the second intake air passage; and a control element for controlling the first communication element in accordance with the operating state of the engine, the air intake structure further comprising a second communication element which, upstream from the first communication element in the flowing direction of intake air, can bring the first intake air passage and the second intake air passage into communication, and the control element being adapted to control the first communication element and the second communication element in accordance with the operating state of the engine.